[PATCH] USB ATM: reduce log spamming

Reduce the number of "unknown vpi/vci" debug messages to (usually) at most
one per-urb, rather than one per-cell.  This is only an issue when (a) many
packets come in but no connection is open; and (b) CONFIG_USB_DEBUG is set.

Signed-off-by: Duncan Sands <baldrick@free.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>